Photodetector imagers, in general, comprise a plurality of photodetectors arranged in either a linear or area array. Along the linear array of the photodetectors or along each column of photodetectors in an area array is a transfer device for carrying the charge collected in the photodetectors to read-out circuitry. One transfer means commonly used is a CCD. The photodetectors commonly used are either a photogate detector or a photodiode detector. Both of these type of photodetectors are used in a variety of applications, but each presents certain tradeoffs in performance.
A photogate photodetector comprises a conductive gate over and insulated from a surface of a semiconductor substrate in which the imager is formed. A bias applied to the gate results in a depletion region in the substrate beneath the gate. Radiation entering the substrate is converted to charge carriers which are collected in the depletion region. The photogates exhibit reset characteristics which are free from lag and noise. However, they do suffer from poor quantum efficiency over much of the visible spectrum and tend to present photoresponse non-uniformity across a device because of gate thickness variation and/or depletion depth variation.
A photodiode photodetector comprises regions of opposite conductivity type in the substrate, one of which can be the substrate, forming a pn junction therebetween. Radiation entering the substrate is converted to charge carriers which are collected in the depletion region formed at the pn junction. Photodiode photodetectors tend to be more uniform than photogate photodetectors and exhibit very good quantum efficiency over the entire visible spectrum. However, they can exhibit lag and reset noise. In addition, both photogate and photodiode photodetectors suffer from time dependent response due to the collapse of the depletion region of the photodetector while photo-generated carriers are accumulated. That is, as the charge carriers are collected in the depletion region of the photodetector the depletion region becomes smaller until it is completely filled up.
It is desirable to have a photodetector which not only has good uniformity and good quantum efficiency, but also has time independent response and anti-blooming capability.